A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Before exposure of the substrate to transfer the pattern onto the substrate, a height level of the substrate may be determined and mapped. A resulting height map of the substrate may be employed to position the substrate with respect to a projection system, for example.
In a conventional lithographic apparatus, the height map may be generated by sampling the height of the substrate at predetermined equidistant positions, e.g. positions lying on a rectangular measurement grid. A height level sensor and the substrate may be moved with respect to each other along a trajectory, which trajectory is selected along the predetermined positions. In the conventional lithographic apparatus, the measurement samples are taken while the substrate and the height level sensor move with respect to each other at a constant velocity. Thus, the height level sensor samples the equidistant measurement positions by sampling at a constant sampling rate. It is noted that in the measuring process the substrate or the height level sensor or both may move.
Before reaching a constant velocity a moving part, e.g. the substrate, first needs to accelerate. While accelerating, no samples are taken. The acceleration is performed over a run-in distance, i.e. a first part of a measurement trajectory. Over the run-in distance, to obtain a full map of the substrate, the height level sensor and the substrate are not positioned with respect to each other such that samples may be taken, i.e. the run-in distance may be positioned outside a substrate measurement area. In a second part of the measurement trajectory, the velocity may be constant. The second part of the measurement trajectory is positioned in the substrate measurement area such that the height level sensor may take samples at the predetermined positions. Next, during a third part of the measurement trajectory positioned outside the substrate measurement area, no samples are taken and the moving part, e.g. the substrate, may decelerate. The above-described sampling method, i.e. accelerating; sampling, while moving at a constant velocity; and decelerating, may be repeated, until a sample has been taken at every predetermined position.
In the above-described method, a run-in distance may be required to reach a predetermined constant velocity before sampling. The moving part needs to move over a moving distance that is larger than a required sampling distance. To decrease a measurement time period, a higher measurement velocity may be required. For a higher measurement velocity, however, a larger run-in distance may be required and therefore the moving distance should be larger. Such a larger moving distance may require additional space in the apparatus, and may require additional time for the completion of the measuring process.